The present invention relates to a frequency domain instrument, and more particularly to frequency selective improvement of the directivity of a return loss bridge.
A return loss bridge is a three-port device in which a coupled port directionally couples a fixed amount of radio frequency (RF) energy from a transmission path between a transmission port and a test port, i.e., the bridge couples some energy to the coupled port from an energy wave traveling along the transmission path in a reverse direction and rejects energy from an energy wave traveling in a forward direction. Therefore the return loss bridge may be used to measure the return lossxe2x80x94reflected power relative to incident powerxe2x80x94of a load terminating the energy wave in the forward direction at the test port. There is a finite difference between the coupled and rejected energy at the coupled port as there is not perfect rejection of the energy wave in the forward direction. This is a figure of merit of the bridge known as xe2x80x9cdirectivity.xe2x80x9d Directivity is defined as the difference, usually in dB, in the power at the coupled port when the test port is terminated in a fully reflected loadxe2x80x94open or shortedxe2x80x94and in a perfect nonreflective load of the same characteristic impedance or at least a load that is much better than the directivity of the bridge.
A schematic for an ideal bridge (without parasitic effects) is shown in FIG. 1. The coefficient k is a trade-off chosen depending upon the amount of coupling desired at the coupled port P3 versus the loss incurred in the transmission path between the transmission port P1 and the test port P2. Energy in the forward direction travels from port P1 to port P2, and energy in the reverse direction travels from port P2 to port P1 with a portion coupled to port P3 by a coupling resistor R0/k. A measurement instrument receiver is connected to port P3 while a measurement instrument transmitter is connected to port P1 and a system under test is connected to port P2. A representative value of k=0.2 provides about xe2x88x9217 dB of coupling (S22) and about xe2x88x922 dB of loss (S21), as shown in FIG. 2. With an ideal balun and no parasitic effects in the circuit the directivity is infinite, i.e., none of the energy wave in the forward direction is coupled to port P3. However a ferrite encased coaxial balun does not provide a perfectly balanced output and causes the directivity to be finite. This finite directivity is defined as an inherent directivity. The inherent directivity is primarily determined by the unbalance provided by the balun formed by the ferrite encased coaxial line assuming no other parasitic effects in the rest of the circuit. If the parasitic effects are kept to a minimum by compact part placement and short line lengths, in particular the length and transitions between port P2 and the coupling resistor R0/k, an inherent directivity of  greater than 30 dB up to very high frequencies, such as 3 GHz, may be achieved. FIG. 3 is a schematic for a return loss bridge with minimal circuit parasitic effects and a balun modeled by an ideal transformer with shunt resistive and inductive parasitics to simulate the unbalance of the ferrite encased coaxial cable.
FIG. 2 is a plot of the simulated results of the circuit of FIG. 3. The directivity is the difference between the loss from port P2 to port P3 (S32) and the loss from port P1 to port P3 (S31). As shown there is a consistent directivity of 30 dB across the frequency sweep up to 3 Ghz. This is the inherent directivity of the bridge with this ferrite encased coaxial cable and no other parasitic effects in the circuit.
However there are situations where, in order to fit the bridge into a specific package, an implementation of the bridge is built that introduces significant parasitic effects. For example where the bridge is built on a circuit board having both sides etched, having via hole transitions, and having a right angle SNA to N cable placed between port P2 and resistor R0/k, all of these discontinuities conspire to degrade the measured or apparent directivity of the bridge. This occurs because the discontinuities in the characteristic impedance of the line cause reflections of the energy wave in the forward direction between port P1 and port P2 before it has a chance to dissipate in the perfect termination at port P2. These reflections couple to port P3 and sum with the energy which is at port P3 due to the inherent directivity caused by imbalance (see FIG. 11). Together the two vectors representing the undesired reflected energy due to parasitic effects and the inherent directivity due to the unbalance of the balun sum to the apparent directivity vector.
To improve the directivity where there are significant parasitic effects, the parasitic effects could be tuned out, depending upon the highest frequency for which the bridge is specified. However at high frequencies, such as 2.5 GHz, it is difficult, if not impossible, to tune two right angle transitions for a return loss on the order of  greater than 30 dB. If it were possible, the apparent directivity would return to the inherent directivity without the discontinuities. Therefore the directivity of the bridge is still limited to the inherent directivity.
What is desired is a method of improving the apparent or inherent directivity of a return loss bridge when there are imperfections introduced by the implementation architecture, especially in a frequency region of interest.
Accordingly the present invention provides a method of frequency selective improvement of the directivity of a return loss bridge by creating a counteractive reflected energy vector at a coupled port that cancels an apparent directivity vector over a desired frequency range. The counteractive reflected energy vector is the vector sum of an inherent directivity vector due to unbalance of a balun and a reflected energy vector from undesired energy reflections caused by parasitic effects in a transmission path between a transmission port and a test port of the bridge. One embodiment inserts an open circuit stub between a coupling resistor and the test port together with a length of transmission line to induce the counteractive reflected energy vector in the transmission path. The magnitude of the counteractive reflected energy vector is determined by the size of the open circuit stub and is made equal to the magnitude of the apparent directivity vector, and the phase is 180xc2x0 opposed to that of the apparent directivity vector as determined by the length of the transmission line between the coupling resistor and the open circuit stub. Alternatively to avoid degrading the return loss of the test portxe2x80x94another figure of merit of the bridgexe2x80x94caused by the first embodiment, energy may be coupled from the transmission path between the coupling resistor and the test port to the coupled port with directional couplers. The coupling coefficient of the couplers determines the magnitude of the counteractive reflected energy vector at the coupled port, which magnitude is set to be equal to that of the apparent directivity vector. The path between the couplers includes a transmission line, the length of which determines the phase of the counteractive energy vector at the coupled port to be in opposition to the phase of the apparent directivity vector. The transmission line and couplers may be enhanced with PIN diodes to provide tunability. In this case the coupling coefficient provides a counteractive vector whose magnitude is greater than the apparent directivity vector, and the PIN diodes are tuned to adjust the counteractive reflected energy vector magnitude and phase at the coupled port to provide improved directivity over the desired frequency range.
The objects, advantages and other novel features of the present invention are apparent from the following detailed description when read in conjunction with the appended claims and attached drawing.